1. Technical Field
This disclosure relates to a non-contact electric power transmitting device and an electric power transfer system, and more particularly, to an electric power control technology in a non-contact electric power transmitting device that transmits electric power to an electric power receiving device in a non-contact manner.
2. Description of Related Art
Japanese Patent Application Publication No. 2014-207795 (JP 2014-207795 A) discloses a non-contact electric power feed system that feeds electric power from an electric power feeding device (an electric power transmitting device) to a vehicle (an electric power receiving device) in a non-contact manner. With this non-contact electric power feed system, the electric power feeding device is equipped with an electric power transmitting coil, an inverter and an electronic control unit. The electric power transmitting coil transmits electric power in a non-contact manner to an electric power receiving coil that is mounted in the vehicle. The inverter generates an AC current corresponding to a drive frequency, and outputs the generated AC current to the electric power transmitting coil. The electronic control unit acquires a charging electric power command to supply electric power to a battery and an output electric power to be supplied to the battery from the vehicle side, and performs feedback control of the drive frequency of the inverter such that the output electric power follows the charging electric power command.
Then, in this non-contact electric power feed system, when the supply of electric power from the electric power feeding device to the vehicle is started, an initial frequency is set based on a state of the battery and a coupling coefficient between the coils (the electric power transmitting coil and the electric power receiving coil). The aforementioned feedback control is started using the initial frequency as an initial value of the drive frequency (see Japanese Patent Application Publication No. 2014-207795 (JP 2014-207795 A)).
In the case where the inverter is a voltage-type inverter and supplies a transmitted electric power (i.e. an electric power to be transmitted to the electric power receiving device) corresponding to the drive frequency to an electric power transmitting unit, the transmitted electric power can be controlled by adjusting the duty cycle of an output voltage of the inverter. In addition, a turn-on current representing an output current of the inverter at the time when the output voltage of the inverter rises can be controlled by controlling the drive frequency of the inverter.
In the voltage-type inverter, it is known that when an output current having the same sign as an output voltage (a positive turn-on current) flows through the inverter as the output voltage rises, a recovery current flows through a recirculation diode of the inverter. When the recovery current flows through the recirculation diode, the recirculation diode generates heat and causes an increase in loss. Thus, the loss resulting from the recovery current can be kept small by controlling the drive frequency of the inverter to control the turn-on current to or below 0.
However, when the drive frequency of the inverter is changed to control the turn-on current, the frequency of the electric power transferred from the electric power transmitting unit (the electric power transmitting coil) to an electric power receiving unit (the electric power receiving coil) changes, and the efficiency of electric power transfer between the electric power transmitting unit and the electric power receiving unit may decrease.